Adjusting The Sound's Pan
Pan is the volume balance between the left and right channel (speaker).  You can pan a sound so that it sounds like it is moving from the left to right or vice versa.  This is what makes stereo more interesting to listen to.

To edit the pan of a sound, go to the toolbar and make sure that Pan is selected.

To pan a sound, edit a point by dragging it up or down.  Like volume, you can add as many envelope points as you'd like.  

The image below shows a sound panned from left to right.  A envelope point panned all the way to the left channel would be at the bottom of the sound.  An envelope point panned all the way to the right would be at the top of the sound.

Pan Range
Pan can be set to values from 100% left to 100% right.  For better precision while dragging envelope points, you can resize the track.

Keyboard Modifiers
Hold the Ctrl key down to add points anywhere on the sound. 
Hold the Shift key down and click down on an envelope line to move the line up or down.
Hold the Alt key down and click points to remove or delete them.

Setting Exact Volume
If the desired pan cannot be set via mouse dragging, you can set it by right clicking on the point and choosing Edit Exact Value...  This brings up a dialog box that lets you set the exact value.

Fades, Boosts and Reductions
These are shortcuts that add multiple envelope points at a time.  Select an area of a sound and choose Fade In, Fade Out, Reduce, or Boost from the Sound menu.  Fades go from left to right or vice versa over time.  Boosts increase the pan to the right channel for a selected period of time.  Reductions lower the pan to the left channel for a selected period of time.

For tips on boosting or reducing, see How To Boost Or Reduce Parts Of Sounds.
For tips on creating fade ins and fade outs, see How To Add Volume And Pan Fades On Sounds.